than cellular, but most Internet capable mobile devices now come with Wi-Fi capability. There are already millions of installed Wi-Fi networks mainly in congested areas such as airports, hotels and city centers and the number is growing rapidly. Wi-Fi networks are very fragmented but recently there have been efforts to consolidate them. The consolidation of Wi-Fi networks is proceeding both through a community approach, Fon as the prime example, and by the consolidation of Wi-Fi network operators. Wi-Fi Wi-Fi offloading is an emerging business domain with multiple companies entering to the market with proprietary solutions. As standardization has focused on degree of coupling between the cellular and Wi-Fi networks, the competing solutions can be classified based on the minimum needed level of network interworking. Besides standardization, research communities have been exploring more open and programmable design in order to fix the deployment dilemma.<ref>X. Kang et al. ``Mobile Data Offloading Through A Third-Party WiFi Access Point: An Operator's Perspective, in IEEE Transactions on Wireless Communications, vol. 13, no. 10, pp. 5340-5351, Oct. 2014.</ref>A. Y. Ding, et al., Vision: Augmenting WiFi Offloading with An Open-source Collaborative Platform Proc. ACM MobiCom MCS 2015 A further classification criterion is the initiator of the offloading procedure. Cellular and Wi-Fi network interworking Depending on the services to be offloaded and the business model there may be a need for interworking standardization. Standardization efforts have focused on specifying tightly or loose coupling between the cellular and the Wi-Fi networks, especially in a network-controlled manner. 3GPP based Enhanced Generic Access Network () architecture applies tight coupling as it specifies rerouting of cellular network signaling through Wi-Fi access networks. Wi-Fi is considered to be a non-3GPP WLAN radio access network (RAN). 3GPP has also specified an alternative loosely coupled solution for Wi-Fi. The approach is called Interworking Wireless LAN (IWLAN) architecture and it is a solution to transfer IP data between a mobile device and operator's core network through a Wi-Fi access. In the IWLAN architecture, a mobile device opens a VPN/IPsec tunnel from the device to the dedicated IWLAN server in the operator's core network to provide the user either an access to the operator's walled-garden services or to a gateway to the public Internet. With loose coupling between the networks the only integration and interworking point is the common authentication architecture. The most straightforward way to offload data to the Wi-Fi networks is to have a direct